Induction motors and universal motors are currently being used in most applications requiring constant-speed and low horsepower, mainly because of their competitive cost. To replace such conventional motors, research has been conducted on single-phase switched reluctance machines (“SRM”) over the last decade. However, prior single-phase SRM machines are not generally suitable for high performance applications since they are known to have some inherent limitations, including low output power density and only a 50% duty cycle of torque generation. They also require an additional component in the form of permanent magnets or auxiliary windings for self-starting.
Because of the known deficiencies of single-phase SRMs, there has been more attention paid to multi-phase SRM machines (i.e., having more than one phase), especially for high torque and/or high-efficiency applications. For example, two-phase SRMs may be employed as brushless motor drives in variable-speed applications, such as those found in home appliances and power tools. Two-phase SRMs are particularly desirable because of their relative simplicity in design and lower costs to manufacture. Various types of two-phase SRMs are known in the art, for example, as described in U.S. Pat. No. 7,015,615, by K. Ramu et al., issued Mar. 21, 2006.
FIGS. 1A and 1B illustrate one example of a conventional two-phase SRM 100. The exemplary two-phase SRM includes a stator 110 having four stator poles 115 and a rotor 120 having two rotor poles 125. The rotor 120 is adapted to rotate around a fixed shaft 130 connected to the center of the rotor. A first pair of concentric windings 140, such as copper coils, are positioned around respective diametrically opposite stator poles 115A. The windings 140 may be electrically connected in series or in parallel. Similarly, a second pair of concentric windings 150 is positioned around respective diametrically opposite stator poles 115B. The windings 150 likewise may be connected in series or in parallel. FIG. 1A shows the exemplary two-phase SRM 100 in a first phase. In this first phase, a current is applied through the windings 140 and the resulting magnetic forces cause the rotor poles 125 to align with the stator poles 115A. FIG. 1B shows a second phase in which a current through the windings 150 causes the rotor poles 125 to align with the stator poles 115B. By selectively energizing the windings 140 and 150, the first and second phases of the SRM are activated and the rotational speed of the rotor 120 can be controlled.
The phase windings in a multi-phase SRM are typically energized by a control circuit associated with the SRM. As used herein, a “phase winding” refers to one or more windings used to activate a single phase of a SRM or other brushless machine. For example, in FIGS. 1A and 1B each set of windings 140 and 150 may constitute a different phase winding in the SRM 100. Most typically, the SRM control circuit comprises at least one switch per phase winding, for turning on and off current flow in that winding. For example, again with reference to FIGS. 1A and 1B, at least one switch (not shown) may be used to control the current flow through phase winding 140, whereas at least one different switch (not shown) may control the current flow through phase winding 150. U.S. Pat. No. 7,271,564, by K. Ramu, issued Sep. 18, 2007, at FIGS. 1-4 illustrates various examples of conventional multi-switch control circuits for use with multi-phase SRM machines.
One drawback to conventional multi-switch SRM control circuits is their cost. That is, each switch in the control circuit is typically associated with additional circuitry for controlling its operation. For example, each switch may be implemented as a transistor switch having associated circuitry for changing the state of the switch, and may be further associated with other circuit components, such as diodes, resistors, capacitors, etc. In addition, because each switch in the multi-switch circuit may be independently controlled, yet additional circuitry may be required to implement separate switch control strategies. The added circuitry associated with each of the multiple switches tends to significantly increase both the cost and complexity of the SRM control circuit.
To overcome the disadvantages of multi-switch control circuits, single-switch control circuits have been proposed for use with multi-phase SRM machines. Previously known single-switch circuits typically require less circuitry, such as fewer transistor switches and diodes, than conventional multi-switch control circuits. As a result, the single-switch control circuits can reduce both the cost and complexity of the SRM. Such single-switch circuits also have the advantage that they do not require multiple control strategies for controlling multiple switches. Rather, only one switch may be actively controlled to trigger multiple phases of the SRM. Various single-switch SRM control circuits are disclosed, for example, in U.S. Pat. No. 7,271,564, by K. Ramu, issued Sep. 18, 2007.
FIG. 2 illustrates an exemplary single-switch control circuit 200 that can be used in a two-phase SRM. A similar single-switch control circuit is disclosed in U.S. Pat. No. 7,271,564, by K. Ramu, issued Sep. 18, 2007, for example, at FIG. 10. The exemplary control circuit 200 includes a direct current (“DC”) power source 210 and control circuitry 220. As shown, the DC power source 210 may comprise an alternating current (“AC”) voltage supply 215, a full-bridge rectifier (diodes D1, D2, D3, and D4), and a source capacitor C1. The source capacitor C1 may be polarized, so as to maintain a substantially DC (i.e., constant) voltage level between its positive terminal (“positive rail”) and negative terminal (also referred to as a “negative rail,” “common,” or “ground”). Those skilled in the art will appreciate that other types of power sources that supply a substantially constant voltage level and current source for use as a DC power source alternatively could be substituted, e.g., using half-bridge rectifiers or DC voltage supplies, such as batteries.
The control circuitry 220 includes, among other things, a “main” phase winding L1 and an “auxiliary” phase winding L2, both having positive terminals electrically connected to the positive rail of the DC power source 210. The negative terminal of the main phase winding L1 is electrically connected to the collector terminal of a transistor switch Q1 and to an anode terminal of a diode D5. The negative terminal of the auxiliary phase winding L2 is electrically connected to a positive terminal of an auxiliary capacitor C2 and to a cathode terminal of the diode D5. In this context, current enters a phase winding through its positive terminal and exits the phase winding through its negative terminal. The auxiliary capacitor C2 may be a polarized capacitor having the same polarity as the source capacitor C1. For instance, the negative terminal of the auxiliary capacitor C2 may be electrically connected to the negative terminal of the source capacitor C1.
The main and auxiliary phase windings may be positioned on respective pairs of stator poles 115A and 115B (such as the windings 140 and 150 shown in FIGS. 1A and 1B). Although the phase windings L1 and L2 may be spatially separated from the control circuitry 220, and in some cases may be considered to form part of the SRM motor rather than part of its control circuitry, these windings are illustrated in the control circuitry 220 for purposes of discussion.
When current flows through the main phase winding L1, a first phase of the two-phase SRM can be activated. The second phase may be activated when current flows through the auxiliary phase winding L2. When current flows through either of the phase windings L1 or L2, i.e., and thus “energizes” the winding, the resultant magnetic energy effects a positive or negative torque in the SRM, depending on the position of the rotor 120 with respect to the energized winding. For instance, if the rotor poles 125 are rotating toward the energized winding's stator poles, the change in inductance at the stator poles is positive, thus producing a positive “motoring” torque that is output by the SRM. On the other hand, if the rotor poles 125 are moving away from the energized winding's stator poles, the inductance slope is negative and a negative “regenerative” torque will be produced, i.e., sending energy back to the DC source capacitor C1.
In operation, the transistor switch Q1 directs current through either the main phase winding L1 or the auxiliary phase winding L2 and, as such, selects a desired phase activation for the SRM. As shown in this exemplary embodiment, the transistor switch is implemented with an NPN bipolar junction transistor whose emitter terminal is electrically connected to the common (ground) potential and whose collector terminal is connected to the main phase winding L1 and diode D5. The transistor switch is turned ON and OFF by a control signal applied to its base terminal. Additional control circuitry, such as a microprocessor, digital signal processor, application specific integrated circuit, field programmable gate array, etc., for supplying the control signal is not shown but will be familiar to those skilled in the art.
When the transistor switch Q1 is turned ON, the DC voltage from the source capacitor C1 is applied across main phase winding L1 and transistor switch Q1, causing current to flow through the main phase winding and transistor switch. The voltage drop across the conducting transistor switch Q1 is typically negligible compared with the DC source voltage level. While the transistor switch Q1 is turned ON, any current in the auxiliary phase winding L2 will rapidly decay because the auxiliary capacitor C2 discharges to the DC voltage source capacitor C1, thus causing the voltage at the auxiliary capacitor C2 to eventually equal the voltage at source capacitor C1, i.e., resulting in zero voltage across the auxiliary phase winding L2. The auxiliary capacitor C2 may have a relatively small capacitance compared with DC source capacitance C1 to ensure that it can quickly discharge to the DC voltage source 210 and attain the DC source voltage level.
In such a conventional single-switch control example, when the current through the main phase winding L1 exceeds a predetermined level, or some other criteria is satisfied, the control signal applied to the transistor switch may be adjusted to turn OFF the transistor switch Q1. In this case, the current through the main phase winding L1 is redirected through the diode D5, which becomes forward biased when the transistor switch Q1 stops conducting. The redirected current quickly charges the auxiliary capacitor C2 above its residual voltage, i.e., which is equal to the DC source voltage, until the auxiliary-capacitor voltage exceeds the DC source voltage and causes current to flow through the auxiliary phase winding L2.
In some applications, conventional single-switch control circuits may underutilize the torque-producing capability of the SRM. For example, there may exist situations where the auxiliary capacitor C2 generates a current in the auxiliary phase winding L2 before current has finished flowing in the main phase winding L1. In such a situation, simultaneous current flow through the main and auxiliary phase windings may reduce the net torque produced by the SRM, because the auxiliary phase winding L2 may produce a positive torque at the same time that the main phase winding L1 generates a negative torque (or vice versa). A further reduction in net torque may result if the current redirected into the auxiliary phase winding L2 circulates back into the main phase winding L1 or into the source capacitor C1. In these cases, the auxiliary phase winding L2 is deprived from using all of the energy transferred to it from the main phase winding L1, thus reducing the amount of torque that the auxiliary phase winding L2 can produce in the SRM. Such recirculation losses also may increase the commutation time required to transition from the first phase to the second phase.